Abstract
Ontake Volcano is located in central Japan, 200 km northwest of Tokyo and erupted on September 27, 2014. To study the structure of Ontake Volcano and discuss the process of its phreatic eruption, which can help in future eruptions mitigation, airborne electromagnetic (AEM) surveys using the grounded electrical-source airborne transient electromagnetic (GREATEM) system were conducted over Ontake Volcano. Field measurements and data analysis were done by OYO Company under the Sabo project managed by the Ministry of Land, Infrastructure, Transport and Tourism. Processed data and 1D resistivity models were provided by this project. We performed numerical forward modeling to generate a three-dimensional (3D) resistivity structure model that fits the GREATEM data where a composite of 1D resistivity models was used as the starting model. A 3D electromagnetic forward-modeling scheme based on a staggered-grid finite-difference method was modified and used to calculate the response of the 3D resistivity model along each survey line. We verified the model by examining the fit of magnetic-transient responses between the field data and 3D forward-model computed data. The preferred 3D resistivity models show that a moderately resistive structure (30–200 Ω m) is characteristic of most of the volcano, and were able to delineate a hydrothermal zone within the volcanic edifice. This hydrothermal zone may be caused by a previous large sector collapse.
Highlights
Electrical conductivity of the subsurface which is sensitive to porosity, water content, salinity, temperature and enhance by the presence of clay minerals together with considerable heterogeneity that exists both vertically and laterally are important for imaging and monitoring active volcanoes and hydrothermal systems (Revil et al 2002)
They highlighted the advantages of the system for investigating deeper structures and the possibility of identifying resistivity structure responses from sensor heights of 100 m or more, which is important for improving the safety of airborne electromagnetic (AEM) surveys
The results reveal that low-resistivity structures (
Summary
Electrical conductivity of the subsurface which is sensitive to porosity, water content, salinity, temperature and enhance by the presence of clay minerals together with considerable heterogeneity that exists both vertically and laterally are important for imaging and monitoring active volcanoes and hydrothermal systems (Revil et al 2002). Auken et al (2009) used a helicopter-borne transient electromagnetic (TEM) method (SkyTEM) to survey the volcanic terrain of the Galápagos Islands, Ecuador. Their results revealed three resistivity structures: highly resistive basalt, a thin layer with intermediate resistivity on the leeward mountain side which they interpreted as a colluvial deposit and a low-resistivity layer found around the island and at intermediate depth on the windward mountain side. These low-resistivity structures are in the range of freshwater-saturated volcanic rocks
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